The central hypothesis of this proposal is: that the different effects on activity, growth, fattening, and reproduction elicited by time-of-day of meal feeding are produced to a great degree by alterations in the phase relationships of endogenous hormone rhythms; that the changes in physiological postures of activity (migration), growth, fattening, and reproduction noted in vertebrates with seasonal cycles are similarly produced; and that, therefore, meal feeding (specifically the timing of meal feeding) may be an excellent tool to manipulate and examine the phase relationship of the circulating rhythms of prolactin, cortisol, and thyroxine as they relate to activity, growth, fattening, and reproduction. This study will examine the interrelationship of the daily activity rhythm; the daily thythms of cortisol, prolactin and thyroid hormones; lipid and nonlipid weight gain; and reproduction in fishes in response to meal feeding. Fish will be subjected to a feeding regime wherein they receive a single daily meal at a specific time of day. Variables to be examined for fish on each feeding regime include: growth (weight gain and increase in standard length), fattening (chloroform/methanol extracted lipids), activity (remote-monitored), reproductive state (gonadal-somatic index and histology), and circulating hormone titers (radioimmunoassay). The results will add data important to an understanding of the circadian mechanisms involved in the response of vertebrates to a meal-fed diet as well as an understanding of the circadian mechanisms involved in growth, fattening and reproduction. In addition, data collected in this study will allow an examination of the potential of timing the daily meal as a tool in aquaculture to increase growth rate and food conversion efficiency.